Control of the Electronic State of Co Oxides and Studies on Novel Superconductivity

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Many systems with strongly correlated electrons have been synthesized and their physical properties have been studied to find "unusual" or "anomalous" physical properties which are useful for development of novel physical concepts. In the studies, we synthesized materials and characterized them first, and then, studied their macroscopic properties. If they seemed to have physically attractive nature, experimental methods, such as NMR and neutron scattering, were applied to further study it microscopically, where knowledge accumulated in the past two decades in the study of high To Cu oxides were used as the guide line of the works. In the microscopic studies, we often used single crystals prepared by ourselves, which made us possible to obtain quite reliable experimental results. The main subjects on which we could make significant achievements are ; (1) several problems which remained in the studies of high-T, superconductivity of Cu oxides, (2) mechanism of the superconductivity of N … More a_xCo0_2 yH_2O (y〜3), (3) unusual behavior of the anomalous Hall effect observed in nontrivial magnetic structures such as Nd_2Mo_2O_2 and spin-glass system, (4) spin liquid state of the quantum spins in pyrochlore system Hg_2Cu_2F_6S, (5) control of the spin state of Co ions in perovskite and related systems, (6) a spin gap phenomenon and a new type phase transition found in(pseudo)honeycomb systems, (7) search for multiferroics and detailed studies of multiferroic phenomenon found in systems with CuO_2 ribbons having simple spin 1/2 Cu^2+ chains, and so on. In the studies of these systems, preparation or syntheses of the systems and finding new physical phenomena are closely connected. For the further description of the achievements, we add the following as the examples. In the study of the superconductivity, we have shown that the Cooper pairs are in the singlet state, excluding predictions of many theoretical works as well as results of experimental studies carried out in the same period by other groups. We have presented complete T_c-v_Q phase diagram (v_Q : NMR quadrupole frequency), and also obtained key information on the dynamical magnetic behavior by neutron inelastic scattering of the superconductor. On the controversy if Nd_2Mo_2O_7 is the unique example of systems which exhibit the unusual anomalous Hall effect induced by the spin chirality mechanism, we have excluded such the possibility quite clearly by detailed neutron diffraction studies. To find the realization of the chirality mechanism of the anomalous Hall effect at least in periodic system remains as the future subject Our discovery of the multiferroic system LiVCuO_4 has presented a new attractive example with spin 1/2 quantum spin chains 'with helical structure, which is simple and convenient for the detailed study on the characteristics of the multiferroic behavior Less